An evaluation of nitroblue tetrazolim dye reduction and hexose monophosphate shunt activity of white blood cells has been undertaken in selected groups of children. Study groups include: (1) premature and full term (normal and infected) neonates; (2) study of children with acute lymphocytic leukemia and with solid tumors during remission, relapse and infection; and (3) a 3 year, double blind study of the value of the NBT dye test in differentiating acute from chronic infection in patients with cystic fibrosis. In addition, a study of factors affecting NBT dye reduction in vitro has been undertaken. To date, these parameters have been evaluated in 114 healthy full term infants, 34 healthy premature infants, 54 infants with suspected sepsis and 13 infants with documented sepsis. These studies also have been performed upon blood obtained upon 414 occasions from 106 patients with acute lymphocytic leukemia or solid tumors during periods of remission and relapse, and in the presence and absence of infection. Five patients who are renal transplant recipients have been sampled following transplantation during periods of rejection and infection.